The quantity of organic matter in sediments is determined indirectly, usually by multiplying by an appropriate factor, some property of the sediment that is related to the organic content--such as the content of carbon, nitrogen, phosphate, or volatile substances; or the loss on ignition; or the texture of the sediments.

The quantity and nature of the organic matter that is deposited are intimately related to the environmental conditions of deposition. A large number of factors are involved, many of which are interdependent. The three main factors are: (1) the supply of organic matter in the overlying water; (2) the rate of decomposition of the organic substances while they are in the water or after they have accumulated in the sediments; and (3) the movement of the water in which the materials are deposited. The supply of organic matter in typically marine deposits depends chiefly upon growth of the plant plankton in the water, which in turn depends primarily upon the supply of sunlight and mineral nutrients. The supply of nutrients is influenced by the movement or circulation of the water, which in urn is affected by other factors. The decomposition of the organic matter depends upon many things, but mainly upon the supply of oxygen. The supply of oxygen is influenced chiefly by the amount of organic matter that is decomposed and by the circulation of the water, which may or may not bring the water to the surface where the oxygen can be replenished. The deposition of the organic matter depends mainly upon the movement of the water. If currents are strong, little organic matter is deposited and if they are weak, much is laid down.

The configuration of the sea bottom, even in deep water far from shore, influences considerably the movement of the water. The organic content of the sediments accordingly is very closely related to the submarine topography. It is relatively small on ridges, which are exposed to currents, and it is considerable in basins, which are protected. As the texture of the sediments likewise is influenced by the movement of the water, the organic content is related to the texture. Sands contain relatively little and clays contain considerable organic matter.

The supply of organic matter in the sea varies considerably in different areas, but the average amount, according to available data, seems about 1,000 grams a year per square meter, of which from 0.02 to 2 per cent is deposited in sediments; and about one-seventh of the organic matter that reaches the sea floor is destroyed before the deposits have been buried do a depth of 1 foot. The loss in organic matter while being deposited in sediments is associated with a decrease in content of oxygen and easily decomposable compounds--such as carbohydrates and simple proteins--and the formation of resistant residues rich in lignins.

The organic content of sediments in the open ocean is much less than near shore. The average quantity in deposits far from land is less than 1 per cent, whereas near shore it is about 2.5 per cent. The quantity is relatively low on the continental platform and in deltaic regions, where currents are relatively strong; and it is comparatively high in basins, in some lagoons draining swamps, and in areas where upwelling water rises to the surface and causes a profuse growth of plankton. It is especially high in bodies of stagnant water where hydrogen sulphide forms in the deep layers, such as many Norwegian fiords and the Black Sea. In these areas the organic content of the sediments may be as much as 10 per cent.

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